1. Field of the Invention
The present invention relates to chemical mechanical planarization (CMP) techniques and, more particularly, to the efficient, cost effective, and improved CMP operations.
2. Description of the Related Art
In the fabrication of semiconductor devices, there is a need to perform chemical mechanical planarization (CMP) operations. Typically, integrated circuit devices are in the form of multi-level structures. At the substrate level, transistor devices having diffusion regions are formed. In subsequent levels, interconnect metallization lines are patterned and electrically connected to the transistor devices to define the desired functional device. As is well known, patterned conductive layers are insulated from other conductive layers by dielectric materials, such as silicon dioxide. As more metallization levels and associated dielectric layers are formed, the need to planarize the dielectric material grows. Without planarization, fabrication of further metallization layers becomes substantially more difficult due to the variations in the surface topography. In other applications, metallization line patterns are formed in the dielectric material, and then, metal CMP operations are performed to remove excess metallization.
A CMP system is typically utilized to polish a wafer as described above. A CMP system typically includes system components for handling and polishing the surface of a wafer. Such components can be, for example, an orbital polishing pad, or a linear belt polishing pad. The pad itself is typically made of a polyurethane material or polyurethane in conjunction with other materials such as, for example a stainless steel belt. In operation, the belt pad is put in motion and then a slurry material is applied and spread over the surface of the belt pad. Once the belt pad having slurry on it is moving at a desired rate, the wafer is lowered onto the surface of the belt pad. In this manner, wafer surface that is desired to be planarized is substantially smoothed, much like sandpaper may be used to sand wood. The wafer may then be cleaned in a wafer cleaning system.
FIG. 1A shows a linear polishing apparatus 10 which is typically utilized in a CMP system. The linear polishing apparatus 10 polishes away materials on a surface of a semiconductor wafer 16. The material being removed may be a substrate material of the wafer 16 or one or more layers formed on the wafer 16. Such a layer typically includes one or more of any type of material formed or present during a CMP process such as, for example, dielectric materials, silicon nitride, metals (e.g., aluminum and copper), metal alloys, semiconductor materials, etc. Typically, CMP may be utilized to polish the one or more of the layers on the wafer 16 to planarize a surface layer of the wafer 16.
The linear polishing apparatus 10 utilizes a polishing belt 12 in the prior art, which moves linearly in respect to the surface of the wafer 16. The belt 12 is a continuous belt rotating about rollers (or spindles) 20. The rollers 20 each have a plurality of parallel grooves 30 where a groove direction is parallel to the polishing belt 12 travel direction. The rollers are typically driven by a motor so that the rotational motion of the rollers 20 causes the polishing belt 12 to be driven in a linear motion 22 with respect to the wafer 16. Typically, the polishing belt 12 has seams 14 in different sections of the polishing belt 12.
The wafer 16 is held by a wafer carrier 18. The wafer 16 is typically held in position by mechanical retaining ring and/or by vacuum. The wafer carrier positions the wafer atop the polishing belt 12 so that the surface of the wafer 16 comes in contact with a polishing surface of the polishing belt 12.
FIG. 1B shows a side view of the linear polishing apparatus 10. As discussed above in reference to FIG. 1A, the wafer carrier 18 holds the wafer 16 in position over the polishing belt 12. The polishing belt 12 is a continuous belt typically made up of a polymer material such as, for example, the IC 1000 made by Rodel, Inc. layered upon a supporting layer. The support layer is generally made from a firm material such as stainless steel. The polishing belt 12 is rotated by the rollers 20 which drives the polishing belt in the linear motion 22 with respect to the wafer 16. In one example, an air bearing platen 24 supports a section of the polishing belt under the region where the wafer 16 is applied. The platen 24 can then be used to apply air against the under surface of the supporting layer. The applied air thus forms an controllable air bearing that assists in controlling the pressure at which the polishing belt 12 is applied against the surface of the wafer 16.
FIG. 1C shows an overhead view of the rollers 20 in the linear polishing apparatus 10. During the CMP process, liquid substances such as, for example, slurry or aqueous substances may be applied. Consequently, liquids may come between the rollers 20 and the polishing belt 12 (as shown by the dotted lines). When this happens, hydroplaning may be occur resulting in slippage between the polishing belt 12 and the rollers 20. Such slippage may result in inaccurate and inconsistent polishing of the wafer 16. To help reduce this problems, the rollers 20 have a plurality of parallel grooves 30 that enable liquids to be removed from the contact areas between the rollers 20 and the polishing belt 12. Each of the plurality of parallel grooves 30 are parallel to each other and non-spiraling. Unfortunately, due to each one of the plurality of grooves 30 forming separate, distinct, and unconnected rings around the rollers 20, certain portions of the polishing belt 12 is always over one of the plurality of grooves and is not supported during the rolling of the polishing belt 12.
Because the grooves 30 on the rollers 20 are parallel, the center of the wafer polishing position is lined up with groove sections on both rollers. Without a rigid support such as a stainless steel band, the polishing belt 12 is not evenly stretched across the roller surface. The uneven tension profile directly transfers to the uneven polishing pressure on the belt 12. Because of the parallel pattern on the rollers 20, when the belt 12 is rolling during polish, the uneven tension pattern does not change across the belt 12 (perpendicular to the belt travel direction) at any given time. As the wafer 16 spins, this effect may average out. However, even with the wafer spinning, the center of the wafer 16 always xe2x80x9cseesxe2x80x9d low pressure, therefore, the removal is the lowest at wafer center.
FIG. 1D shows uneven polish profile is directly transferred to the uneven polishing pressure on the polishing belt 12. The y-axis is a polishing pressure and the x-axis is distance from the center of the wafer 16. Curve 32 shows the distance from the radius of the wafer 16 plotted against the polishing pressure. Because of the parallel pattern on the rollers 20, when the belt 12 is rolling during polishing, the uneven tension pattern does not change across the belt. Due to the lack of support over areas of the plurality of parallel grooves 30, a set of concentric rings separated by, in one example, 0.5xe2x80x3 is observed on a polished wafer. Generally, oscillation in removal rate from wafer center to edge corresponds to the rings visually detected on the polishing belt 12. Even if the wafer is spun during polishing, the center of the wafer 16 typically has a minimal polishing rate compared to other areas of the wafer 16.
Therefore, different sections of the polishing belt 12 may have differing tensions which may result in differing polishing rates on certain portions of the wafer 16. Consequently, wafer processing may be less consistent and more wafers may be damaged.
Broadly speaking, the present invention fills these needs by providing an improved apparatus for rotating a polishing belt in a linear chemical mechanical planarization (CMP) process. The apparatus includes spiral grooves and lateral channels defined on an outside surface of a roller that rotates a polishing belt during CMP. It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device or a method. Several inventive embodiments of the present invention are described below.
In one embodiment, in a linear chemical mechanical planarization (CMP) system which includes a linear belt and a pair of rollers where the linear belt loops around each of the pair of rollers and the pair of rollers is designed to drive the linear belt to enable planarization of a substrate, a surface of each roller of the pair of rollers is provided. The surface includes a first set of grooves covering a first portion of the surface of the roller where the first set of grooves has a first pitch that angles outwardly toward a first outer edge of the roller. The surface also includes a second set of grooves covering a second portion of the surface of the roller where the second set of grooves has a second pitch that angles outwardly toward a second outer edge of the roller with the second pitch angling away from the first pitch. The surface further includes a first set of lateral channels arranged along the first portion, and a second set of lateral channels arranged along the second portion. The first set of lateral channels crosses the first set of grooves, and the second set of lateral channels crosses the second set of grooves.
In another embodiment, a method for generating a grooved roller for use a linear chemical mechanical planarization (CMP) system is disclosed including providing a roller. The method also includes forming a first set and a second set of grooves on an outside surface of the roller where the first set of grooves covers a first portion of the surface of the roller and the first set of grooves has a first pitch that angles outwardly toward a first outer edge of the roller. The second set of grooves covers a second portion of the surface of the roller, and the second set of grooves has a second pitch that angles outwardly toward a second outer edge of the roller where the second pitch angles away from the first pitch. The method also includes forming a first set and a second set of lateral channels on an outside surface of the roller where the first set of lateral channels is arranged along the first portion, and the second set of lateral channels is arranged along the second portion. The first set of lateral channels crosses the first set of grooves, and the second set of lateral channels crosses the second set of grooves.
In yet another embodiment, an apparatus for optimizing linear chemical mechanical planarization (CMP) operations is provided. The apparatus includes a cylindrical roller where the cylindrical roller rotates a polishing belt in a CMP system. The apparatus also includes a first set of grooves defined on an outside surface of the cylindrical roller where the first set of grooves has a first groove initiation point at a center portion of the cylindrical roller and spirals around the cylindrical roller at least one time to a first ending point at a first edge area of the cylindrical roller. The apparatus further includes a second set of grooves defined on the outside surface of the cylindrical roller where the second set of grooves has a second groove initiation point at the center portion different from the first groove initiation point of the cylindrical roller. The second set of grooves spirals around the cylindrical roller at least one time to a second ending point at a second edge area different from the first edge area of the cylindrical roller. The apparatus additionally includes a plurality of lateral channels being defined on the outside surface of the cylindrical roller where the plurality of lateral channels extends at an angle from the center portion of the cylindrical roller to an edge of the cylindrical roller. The plurality of lateral channels and the first set and the second set of spiral grooves remove fluid from an interface between the cylindrical roller and the polishing belt when the cylindrical roller rotates the polishing belt, and the first set and the second set of spiral grooves apply a consistent tension pattern across a width of the polishing belt when the cylindrical roller rotates the polishing belt.